the skeleton casters are tricky. the best thing to do is load up on cold resist gear. cold doesn't do much damage to you but, if you get frozen the other casters will wipe you out. Watch out for the lightning bolts too. one isnt bad but it will put a shocked state on you. once you are shocked everything that hits you will do extra damage. the shocked state from lightning is multiplicative too. so if you get hit by lightning several times in succession, you're going to take double, triple, or quadruple damage. its best to just back off long enough for the shocked status to wear off or use a flask to remove it.